On the one hand, I'd say it's probably because they don't recognize it. I know I've been told to report people for all kinds of things (usually feeding - and it's usually a tank of some sort). But when I look at what he's doing from my perspective, he's trying to set engagements or zone people or peel or body-block to protect people or something else, and it just isn't working well.
Other times, I can totally see the troll behavior. So I make it a policy to only report if I personally see the behavior in question, and recognize it as such.
On the other hand, though, I'd have to ask: why does it matter?
The report system is designed to build cases based on the number of reported games rather than the number of reports per game. So for the purpose of case-building, it genuinely doesn't matter how many reports a player gets. So all that really matters is that you report the behavior you see.
As a corollary to that, there's no real benefit to asking for reports in the first place. And, in fact, it can be seen as a type of bullying, which is frowned on, and can earn a chat restriction.